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This invention relates to the measurement of surface contours of an object using optical measurements of light reflected from the surface of the object. More particularly, this invention relates to a method and apparatus for acquiring images used in surface profilometry for such optical measurements. The invention is particularly useful for inspecting solder paste deposited on electronic circuit boards during the circuit board fabrication process.
Circuit boards that carry electronic integrated circuits as well as discrete electronic components are well known. A circuit board substrate is prepared with predetermined conductor paths and pads for receiving the lead of an electronic component such as integrated circuit chips, resistors or capacitors. During the circuit board fabrication process, solder paste bricks are placed onto the board substrate at appropriate positions. The solder paste is usually applied by placing a screen onto the substrate, applying solder paste through the screen openings, wiping the excess solder paste from the screen surface, and removing the screen from the substrate. The circuit board electronic components are then positioned onto the substrate, preferably with a pick and place machine, with leads of the electronic components placed on their respective, appropriate solder paste bricks. The circuit board is passed through an oven after all of the components are positioned on a substrate, to melt the solder paste thus creating an electrical as well as a mechanical connection between the components and the substrate.
The size of the solder paste bricks, and the accuracy with which they must be placed on the substrate, has become increasing smaller and tighter with the increased emphasis on miniaturization in the electronics industry. Solder paste brick heights can be as small as 200 microns in diameter and having a height of 100 microns. The height of the solder paste brick must often be measured to within 1 percent of the designed height. The center to center spacing between two adjacent solder paste bricks may be as low 300 microns. Too little solder paste in one brick can result in the failure to provide an electrical connection between the lead of an electronic component and the pad of the circuit board substrate. Too much paste in one brick can result in bridging and short-circuiting between the leads of a component.
A single circuit board can cost thousands and even tens of thousands of dollars to manufacture. Testing of a circuit board after the fabrication process is complete can detect errors in solder paste placement and component lead connection, but often the only remedy for a faulty board is scrapping of the board. It is accordingly imperative that a circuit board be inspected during the fabrication process so that improper solder paste placement can be detected prior to the placement of the electronic components onto the substrate. Such in-process solder inspection reduces the cost of failure since expensive components have not yet been placed onto the circuit board.
One important characteristic that is measured during solder paste inspection is the height of each solder brick. Once the height is known and the boundaries of the deposit are located, the volume of paste deposits can also be calculated. One way in which the heights of solder bricks have been measured is by using a technique known as phase profilometry. In some types of phase profilometry, the surface profile of an object of interest is computed by projecting at least three phases of a light pattern onto the object and capturing those images, and then computing the surface profile as a function of the three captured images. One particular method and apparatus for phase profilometry using three or more images is disclosed in U.S. Pat. No. 4,641,972 entitled xe2x80x9cMethod and Apparatus for Surface Profilometry.xe2x80x9d However, the method and apparatus of the ""972 patent suffers from susceptibility to system vibration. Such susceptibility renders the system disclosed in the ""972 patent unsuitable for industrial applications in high vibration environments.
Vibration immunity is very important for profilometry systems when they rely upon the surface feature (e.g. solder brick) to be essentially in a known position while sequentially exposed to each of the projected patterns. Automated electronics assembly lines often generate a relatively large amount of vibration. It is generally preferred to locate the solder paste inspection machine within the assembly line such that it inspects solder paste immediately after paste deposition and before component placement. However, the proximity of the solder paste inspection machine to other machines introduces vibrations, which can adversely affect solder paste inspection. For example, when a surface feature on the circuit board (such as a solder brick) vibrates, it appears in one place when imaged with a first projected pattern and appears in other places when imaged with successively projected light patterns, leading to errors in the resulting map of surface heights.
Therefore, it is important to the field of surface profilometry to minimize uncontrolled displacement between successively captured images. If all the required images can be acquired within approximately 1 millisecond or less, the images will be substantially immune to vibration in most industrial environments. The immunity is provided by the fact that the vibrating surface does not travel appreciably in one millisecond.
One method for electronically capturing images for optical inspection uses a frame transfer Charge Coupled Device (CCD) array. One such device is the Model THX 7887A available from Thompson-CSF Semiconducteurs Specifiques of France. A frame transfer CCD array is a photosensitive semiconductor device that includes an image area as well as a memory area thus allowing an image stored in the memory area to be read out, while another image is acquired in the image area. While these conventional arrays can be configured to permit the rapid acquisition of two images, they have been unable to do so for the three images required in advanced systems.
Thus, there is a need for an inspection system and an associated method, which can capture three or more successive images within a period of about 1 millisecond in order to provide an industrial profilometry system that is substantially immune to vibration.
Methods and an apparatus are disclosed for providing enhanced vibration immunity in a solder paste inspection system, although they are usable in any number of industries that require rapid acquisition of several images. The method includes capturing at least three images on a frame transfer CCD array before any data is sequentially read from the array. The present method is extendable to a larger number of images. Additionally, the masked memory area can be larger than the image area of the frame transfer CCD array.